The invention relates to an optical modulator and more particularly to an achromatic attenuator, particularly for guidance and camera lenses.
Numerous different devices exist for attenuating the intensity of light. Thus, there are attenuators with mechanical displacement of optical devices, attenuators by the electrochemical deposition of a metal on the transparent conductive walls of an electrolytic tank, attenuators by reduction or oxidation of thin metal films and attenuators utilizing the effect of an electrical field in a mixture of a liquid crystal and a dichroic dye.
The object of the invention is to propose a modulator using a different process and which can more particularly be used in tracking sights. This type of application requires high transmission dynamics exceeding 20 dB for compensating surrounding illumination variations, which can be sudden, as well as a high transmission in the open state in order to permit the detection of the light reflected by the target. Moreover, the specific technical characteristics of the detection device require a spectral pass band between 0.4 and 1.1 microns and a minimum image distortion.
Within the scope of the invention, it is proposed that use be made of the movements of electrically controlled thin liquid layers without any intervention of movable mechanical means, in order to modify the refraction and absorption conditions encountered by a light beam traversing the attenuation device.
French Patent Application 83.04745, filed by the Applicant Company on Mar. 23, 1983 describes a device making it possible to displace a liquid layer. Such a device comprises a capillary space defined by two confinement plates, whose inner faces are provided with electrodes and make it possible to apply an electrical field to the capillary space. The latter contains two immiscible fluids having different dielectric permittivities. The application of a voltage between the electrodes gives rise to an electrical field attracting the higher permittivity fluid. The physical phenomenon is the well known phenomenon of filling a capacitor with a dielectric liquid, whilst establishing electrical field gradients and an electrostatic energy variation.
The position control of the fluids in the capillary space is obtained on the one hand by the choice of the electrical fields applied and on the other by specific surface treatments making the surfaces of the electrodes more wetting than the surrounding surfaces. For example, in the case of transparent metallic oxide electrodes, such as the indium - tin oxide, etched on glass, there is a deposit of a polymerized monomolecular organosilane coating.
Thus, the invention supplies a system realising the thus described liquid displacement phenomenon.